A valve operation switching unit for improving a volume efficiency of the combustion chambers over a wide operation range by changing at least either the angular interval of opening the intake valves and/or exhaust valves for each cylinder or the lift of the valves is proposed for instance in Japanese patent laid open publication No. 63-16111.
A variable capacity supercharger offering an optimum supercharge pressure over a wide operating range with a high responsiveness by varying the A/R ratio of an exhaust passage leading to a turbine wheel by means of a flap or a plurality of vanes is proposed in Japanese patent laid open publication No. 62-282128.
According to such a variable capacity supercharger, since a supercharge pressure which is suitable for each operating condition can be arbitrarily and accurately obtained, an even further improvement can be achieved particularly by combining a valve operating condition switching unit and a variable capacity supercharger.
In low speed range it is possible to increase the speed of intake flow directed to the combustion chambers by reducing the angular interval of opening the valves and/or the valve lift, but this tends to limit the intake flow rate as the rotational speed of the engine increases. Conversely, by increasing the angular interval of opening the valves and/or the valve lift in high speed range, the volume efficiency of the engine intake improves as the rotational speed of the engine increases. Therefore, if a variable capacity supercharger used in conjunction with a valve operating condition switching unit is controlled in the same way as if it were used for an engine without any such valve operating condition switching unit, it would not be possible to obtain an optimum performance of the engine in all of its operating range.
In particular, since the change in the movement of the valves during each cycle of engine operation will affect conditions of the intake passages (such as the resonance frequency of the intake passage, the volume efficiency of engine intake, etc.), it is advantageous to adapt the mode of controlling the supercharger to such changes. For instance, in an engine using a valve timing adjusting system which switches over valve timing in step-wise fashion according to the change in the rotational speed of the engine, as the rotational speed of the engine is increased, the torque output reaches a peak value and then gradually diminishes before the valve timing is switched over to from a low speed mode to a high speed mode. This decline in the torque output between the point of the torque peak and the point of valve timing switch over may be felt by the operator as a torque dip, and it is desired to remove such a torque dip.
As an additional consideration, such a complex control action should not involve any undue delay as such a delay will seriously impair the commercial value of the vehicle on which the engine is mounted. However, a high responsiveness of an engine must be accompanied by a sufficient control stability.
Further, in view of the complexity of the overall control system, it is desired to have a fail safe feature to be incorporated into the system.